This invention relates generally to testing of coaxial cable and in particular to an interface connector for use during testing, and is more particularly directed toward a test interface providing a temporary, high quality connection between testing apparatus and a length of coaxial cable under test.
Generally, a coaxial cable has an inner central conductor, a dielectric surrounding the central conductor, and an outer conductor coaxial to and surrounding the central conductor. The inner conductor transmits signals and the outer conductor acts as a signal return and as a ground. The outer conductor also provides shielding for the inner conductor.
It is known that semi-rigid cable can be tested to determine if the electrical length of the cable is correct. U.S. Pat. No. 4,441,781 discloses using a test connector to determine if the electrical length is correct. If the length is correct, the test connector is removed and a coaxial connector is permanently terminated onto the coaxial cable.
However, known test connectors do not provide a high quality connection between the cable under test and the test apparatus. Testing multiple reels of flexible coaxial cable requires a connection to the test equipment that is quick, easy, and low cost. It must also provide high-quality electrical performance over a wide frequency range, so that cable performance is not masked by a poor quality test connection. Commercially available connectors with the required performance are designed for permanent installation. They are expensive and not generally reusable, and if an attempt is made to reuse them, it can generally be done only a few times. It is very time consuming to remove and reinstall these commercially available connectors on the next cable to be tested.
Consequently, a need arises for a temporary, reusable test connection between testing apparatus and a section of coaxial cable under test that is economical, easy to connect and disconnect, and that provides a high-quality connection over a wide range of frequencies.
These needs and others are satisfied by the test interface of the present invention. The present invention comprises a new connector design conceptualized by using the type N connector on the test equipment and the coaxial cable only as constraints. The electrical performance is better than known designs of the prior art, having good electrical performance extending to a test frequency of six gigahertz or greater. It provides ease of use with its sliding cable insertion and removal, and is readily reusable over a large number of insertion and removal cycles.
Key design features of this new interface are the length and shape of the connector center conductor (i.e., pin), and the design of the connector body. The new connector body allows the length of the pin to be shortened to the minimum length possible. The pin length, from the end of the socket at which it mates to the end of the pin that contacts the cable center conductor, is reduced to about one-tenth electrical wavelength at the upper frequency of interest.
The end of the pin that contacts the cable does not have the shoulder that was commonplace in older test connector systems. Removing this shoulder eliminates a large impedance variation in the interface, as well as eliminating the resulting poor electrical performance at frequencies above one gigahertz.
In accordance with one aspect of the present invention, a test interface is provided between cable testing apparatus and a coaxial cable for testing the coaxial cable over a frequency range of interest that extends from a lower frequency to an upper frequency. The coaxial cable includes a center conductor, a dielectric portion, and a shield conductor, and the test interface accommodates a prepared coaxial cable end wherein the coaxial cable shield conductor is folded back along its length and the center conductor and dielectric portions are trimmed substantially flush with the fold. The test interface comprises a coupler having a cable lead-in opening at a first end and a threaded, connector contact portion near a second end, an insert that abuts an interior shoulder of the coupler proximate the connector contact portion, the insert having a wall portion that projects outwardly toward the second end of the coupler, and a pin constructed and arranged to abut a center conductor of the coaxial cable, the pin substantially centrally positioned within the outwardly projecting wall portion of the insert. The coupler, insert, and pin mechanically and electrically interconnect with a mating electrical connector to provide a reusable electrical connection between the cable testing apparatus and the coaxial cable.
In one form of the invention, the coupler includes a substantially conical lead-in portion that begins proximate a cable entry end of the coupler, the conical lead-in portion tapering substantially linearly to a relatively constant-diameter cable contact portion. Preferably, the conical lead-in portion tapers substantially linearly at a taper angle of approximately 10 degrees. The insert may have a plurality of cutouts disposed about the perimeter of the wall portion of the insert, in substantially equally spaced relation.
The pin preferably includes a body portion and an end portion, the end portion meeting the body portion at a distinct shoulder, and constructed and arranged to engage a center contact of a mating connector, and the body portion including a mating surface, distal from the end portion thereof, constructed and arranged to abut the center conductor of the coaxial cable. Preferably, the body portion of the pin has a length of about one-tenth wavelength at the upper frequency of interest. The mating connector may be a female type-N connector.
Further objects, features, and advantages of the present invention will become apparent from the following description and drawings.